It has been common practice to include in lubricant formulations additives to provide improved antiwear and rust inhibition properties. In the past, sulfurized triglycerides, such as sulfurized lard oil, have been utilized, especially in association with lightly refined aromatic mineral oils which provided sufficient solubility for the sulfurized triglycerides.
With the increased concern for toxicity of aromatic compounds found in such mineral oils, lubricant formulations now comprise essentially non-aromatic oils. This change to substantially non-aromatic base oils created a major problem, resulting from a significant decrease in solubility of the sulfurized triglycerides in the non-aromatic mineral oil, resulting in solidification and/or dropout of the sulfurized triglycerides.
While the solubility problem has been overcome, the modified lubricant products have been found to be either deficient in desirable lubricant properties or incapable of providing needed improvement in these properties.
In a typical approach to this problem, as reported in U.S. Pat. No. 3,455,896, sulfurized, low molecular weight polybutenes were reacted with liquid triglycerides, which were susceptible of sulfurization, to yield an additive. In U.S. Pat. No. 3,850,825, another additive was prepared by the sulfurization of a mixture of prime burning lard oil and alkyl oleate. In U.S. Pat. No. 3,740,333, C.sub.10 -C.sub.16 alcohol esters of unsaturated fatty acids, having 18 to 22 carbon atoms, were blended with a triglyceride and either used "as is" or sulfurized. Modifications of such compositions have been reported in U.S. Pat. Nos. 4,149,982, 4,166,795, 4,166,796, 4,166,797, and 4,188,300.
Although these prior art efforts have increased the solubility of sulfurized fatty oils to acceptable values, there has remained a serious need for sulfurized additives possessing both good solubility and a combination of improved lubricant properties, such as, for example, better low temperature flow properties, better load carrying and antifriction properties, and a lack of sludging. Such improved lubricant properties would also be attractive for use in various fuels systems employed for power generation and heating purposes.